The present invention relates to a process in which an admixture of toluene and C.sub.9 alkylbenzene is transalkylated to form benzene and C.sub.8 alkylbenzene. It specifically relates to fractionation of the reaction zone effluent of a process in which an admixture of toluene and C.sub.9 alkylbenzene is transalkylated to benzene and C.sub.8 alkylbenzene.
At the present time, about 90 percent of the benzene produced in the U.S. is derived from petroleum sources and the balance is derived from natural gas and coal. This represents a sharp change from as recently as 1957 when the steel and coal industries produced more benzene than did the petrochemical industry. This is in large part due to the high growth rate of benzene demand experienced during the past decade, averaging 12 percent per year during that time, but also reflect a stagnation of benzene production by tar distillers and coke oven operators. About 85 percent of benzene consumption in the U.S. now goes to production of ethylbenzene, phenol, and cyclohexane, while a relatively small amount goes to anilin, maleic anhydride, chlorobenzene and other uses. Ethylbenzene, which consumes 48% of U.S. benzene production, is an intermediate chemical in the production of styrene-butadiene rubber and styrene resins such as polystyrene, both the straight and rubber-modified polystyrenes finding application in consumer products such as packaging, toys, luggage, housewares, etc. About 20% of benzene production is used to prepare phenol, which is produced by various methods, principally by way of cumene as an intermediate, and is itself an intermediate chemical in the production of phenolic resins, which are used in molding applications, polywood bonding, laminating resins, friction materials, thermal insulation, etc. About 17% of benzene production goes to produce cyclohexane, an intermediate in the manufacture of nylon. Because of benzene's use in the production of consumer goods, it is expected that demand will continue strong, but perhaps not at the annual growth rate of 12 percent experienced during the past decade due to benzene's present unit cost more or less quadruple that of the previous decade.
Like benzene, demand for xylene has been strong principally due to increasing demand for paraxylene. Over the past decade, while yearly production of mixed xylene has increased 9 percent, that of orthoxylene has increased 13 percent and that of paraxylene has increased 24 percent. Xylenes are produced almost solely from petroleum with less than 2 percent production from coal tar and coke oven light tars. Xylene isomers together with ethylbenzene as produced from petroleum are normally found as follows as related to the total C.sub.8 aromatic: ethylbenzene 15-25 percent, orthoxylene 15-25 percent, metaxylene 35-45 percent, and paraxylene 12-22 percent. As stated hereinabove, ethylbenzene is used in the production of styrene; orthoxylene is a feedstock in the production of phthalic anhydride while paraxylene is used for polyester manufacture.
Benzene, ethylbenzene, and the xylene isomers are principally prepared and separated together with toluene from petroleum by a series of processing units as follows: (1) In a crude unit, crude petroleum is fractionated into several boiling range cuts, one of which is a naphtha cut which boils in the range of about 100.degree. to 350.degree. F. (2) After depentanizing or deisohexanizing of the naphtha, it is passed to a desulfurization and reforming unit, where sulfur is removed to less than one part per million and the aromatic precursors in the naphtha are upgraded to their respective aromatics. (3) Reforming unit effluent, normally containing about 30 to 60 percent aromatics, is passed into an aromatics extraction unit, wherein normally either a glycol or Sulfolane is utilized as a solvent to extract aromatic components from non-aromatic ones. Extract containing about 99.9 percent aromatics is clay treated to reduce olefin content and is separated in a fractionation zone to prepare the various aromatic components in purified streams as desired. Several important variables affect the quantity of individual aromatic products obtained by the hereinabove processing scheme, the most important of them being the quality of the crude and the market for products. Crudes vary significantly in quality in regard to aromatic and aromatic precursors content, and in regard to the ratio of aromatics and aromatic precursors by carbon number. While there is significant variation, benzene, toluene and xylene (including ethylbenzene) are typically produced in the following ratio:
benzene = 1 PA1 toluene = 2.5 to 3.0 PA1 xylene and ethylbenzene = 2.0 to 2.5
Although it appears that the production rates of toluene and xylene may be greater than that of benzene, actual benzene, toluene, and xylene U.S. production in 1973 were 1453, 936, and 818 million gallons, respectively. Consumption is far greater of benzene than toluene or xylene, and accordingly, production of toluene and xylene is restricted, normally by one of two means. Firstly, a naphtha cut with an end point of about 230.degree.-300.degree. F. may be processed, thereby substantially reducing the toluene and xylene precursors in the reforming unit feedstock; and secondly, a dealkylation unit may be utilized to convert toluene to benzene. A further explanation of higher benzene production than toluene or xylene is related to the aromatics production from coke-oven light oils, coal tar, and pyrolysis processes, which is similar from these sources in distribution of benzene, toluene, and xylene produced. While there may be substantial variation from one producer to another, benzene, toluene and xylene comprise about 80, 15 and 5 percent, respectively, of the BTX aromatics produced in these processes.
Toluene, unlike benzene, orthoxylene, and paraxylene, does not have strong demand as an intermediate chemical in the manufacture of consumer products. End uses such as polyurethane production, aviation gasoline, or solvents require only about 35 percent of U.S. production; the remainder of U.S. toluene production is dealkylated to benzene. In the present description, U.S. production of aromatics stated hereinabove is the production and separation into relatively purified streams of said aromatics, but in fact, total production is substantially greater. For example, only about 20 percent of total toluene produced is actually separated into a purified stream, the remainder being used as a high octane gasoline blending component. As a blending component, toluene has a premium value with a research octane number of 105.8. With the present emphasis on lead-free gasoline, high octane blending components such as toluene are becoming relatively more valuable than previously, but toluene dealkylation has increased and now accounts for about 32 percent of benzene production as compared to about 22 percent in 1965. Accordingly, it is observed that toluene dealkylation is becoming an increasingly attractive route to benzene production.
Both thermal and catalytic toluene dealkylation processes are available to produce benzene. Both catalytic and thermal dealkylation are practiced in the presence of hydrogen at high reaction temperatures to about 1200.degree.-1300.degree. F. to achieve over 95 percent dealkylation to benzene. Both processes may accept feedstock including alkylaromatics higher than toluene, and these are also normally converted to benzene although mild conversion of C.sub.9 and heavier aromatics to xylene is known in the art. Feedstocks including paraffins, naphthenes, and aromatics are also provided to both dealkylation processes to result in benzene and light paraffin products, i.e., methane and ethane. Because of high hydrogen consumption and low benzene volume yields, dealkylation to benzene of alkylaromatics heavier than toluene is not as advantageous as toluene dealkylation.
A relatively new process development is a catalytic transalkylation process in which toluene is transalkylated to benzene and xylene in the presence of hydrogen. The process is advantageous as compared with dealkylation processes in the respect that hydrogen consumption is substantially reduced and reaction temperatures are less severe. In a transalkylation process in which toluene is a feedstock, the principal reaction taking place is as follows: EQU 2 C.sub.7 H.sub.8 .fwdarw.C.sub.6 H.sub.6 + C.sub.8 H.sub.10
molar yields of benzene and xylene are essentially achieved in the process, and while the theoretical volume yield of benzene from toluene is about 84 percent by dealkylation, the combined theoretical volume yields of benzene and xylene is about 100 percent from transalkylation of toluene. Of the C.sub.8 aromatic product, only 1 to 2 percent is ethylbenzene, with the xylene isomers comprising the remainder in the following proportions: para 23-25 percent, meta 50-55 percent and ortho 23-25 percent.
In addition to benzene and C.sub.8 aromatic products, about 2 to 4 percent of the reactant is converted to light hydrocarbons, such as methane and ethane, and heavy aromatics containing 10 or more carbon atoms. An alkylaromatic product containing 9 carbon atoms is produced in the reaction, but following separation of the reaction zone effluent, it may be recycled to extinction in the reaction zone. Although the primary reactant introduced into the process is toluene, C.sub.9 aromatic may also be used and upgraded principally to xylene and a C.sub.10 aromatic. For example, trimethylbenzene is principally converted to xylene and tetramethylbenzene.
The process of the present invention is applicable to a transalkylation process in which an admixture of toluene and C.sub.9 alkylbenzene is transalkylated in a reaction zone wherein complete conversion of the reactants is not achieved, thus requiring separation and recycling to the reaction zone of unreacted reactants.
An object of the present invention is to provide an improved process in which toluene and C.sub.9 alkylbenzenes are transalkylated to provide benzene and xylene product.
A specific object of the invention is to reduce energy consumption in a transalkylation process to convert toluene and C.sub.9 alkylbenzenes to benzene and xylene.
Accordingly, the present invention provides a combination xylene separation and a toluene transalkylation process, for producing benzene and orthoxylene comprising the steps of: (a) transalkylating a mixture of toluene and C.sub.9 alkylbenzene in a transalkylation zone at transalkylation conditions to produce a transalkylation zone effluent comprising benzene, toluene, C.sub.8 alkylbenzenes, C.sub.9 alkylbenzenes, and C.sub.10 alkylbenzenes; (b) fractionating at least a portion of said transalkylation zone effluent in a distillation column wherein benzene and toluene are recovered as an overhead fraction and materials heavier than toluene are recovered as a bottoms fraction; (c) fractionating said benzene and toluene fraction in a fractionation column to produce a benzene product and to recover a toluene-rich stream which comprises at least a portion of the charge to the transalkylation zone; (d) passing at least a portion of said bottoms fraction, comprising material heavier than toluene, into a xylene fractionator, and recovering overhead a stream comprising orthoxylene and recovering as a bottoms fraction a stream comprising C.sub.9 and C.sub.10 alkylbenzenes; (e) passing the C.sub.9 and C.sub.10 alkylbenzene bottoms fraction to a fractionation zone to recover overhead a stream comprising C.sub.9 alkylbenzenes, at least a portion of which C.sub.9 alkylbenzenes stream is charged to the transalkylation zone; (f) reboiling the column producing benzene product by indirect heat exchange with vapors from the column producing C.sub.9 alkylbenzenes as an overhead fraction; and, (g) reboiling the column producing benzene and toluene as an overhead fraction by indirect heat exchange with a stream comprising orthoxylene vapors obtained as an overhead vapor fraction from the xylene fractionator.
In a typical prior art process, the effluent of a toluene transalkylation reaction zone containing benzene, toluene, xylene, C.sub.9 aromatics, and aromatic heavier than C.sub.9 is separated in a fractionation zone into product and recycle streams each of which contains a high purity single carbon number aromatic component. Benzene, xylene, and aromatics heavier than C.sub.9 are withdrawn as product streams, while toluene and C.sub.9 aromatics are recycled as two separate purified streams to the transalkylation zone. In the fractionation zone, reaction zone effluent is typically separated to form benzene as the overhead fraction of a first fractionation, toluene as the overhead fraction of a second fractionator, C.sub.8 aromatics as the overhead fraction of a third fractionator, C.sub.9 aromatics as the overhead fraction and C.sub.10 aromatics as the bottoms fraction of a fourth fractionator. The bottoms fraction of each column is passed as feed to the next column in the series. Each fractionator is provided with an overhead condensation and reflux system and a bottoms indirect reboiler system known to one skilled in the art.
A significant improvement was made over this prior art process when it was discovered that one column could be eliminated, and much expensive heat exchange equipment eliminated by a modification in the flow scheme. Thus, instead of "peeling" off only the lightest component from a multicomponent mixture a more radical fractionation was attempted. Thus, the first fractionator separated the reaction zone effluent to give BT overhead and C.sub.8 to C.sub.10 + alkylaromatics as a bottoms fraction. The C.sub.8 and C.sub.10 + bottoms are sent to a second fractionator. There a C.sub.8 alkylbenzene, orthoxylene, is withdrawn as an overhead product stream while C.sub.9 /C.sub.10 + alkylaromatics are withdrawn as a bottoms fraction. The other C.sub.8 aromatic isomers were sent to an isomerization unit. The BT fraction and the C.sub.9 /C.sub.10 + fraction are passed into a third fractionator, at separate loci of the column, and separated into an overhead product stream containing benzene, a sidecut recycle stream containing C.sub.7 and C.sub.9 aromatics in admixture, and a bottoms product stream containing C.sub.10 + aromatics. Each fractionator was provided with indirect overhead condensation and indirect bottoms reboiler system known to one skilled in the art. The ortho-xylene product, recovered as an overhead fraction, was used to reboil the fractionator downstream of the transalkylation unit. Benefits of this processing scheme included: (1) elimination of a fourth fractionator including an overhead condensation system and a bottoms reboiler system, and (2) reduction of utility requirement, as C.sub.9 vapors provide heat to vaporize benzene for the benzene-toluene split. This flow scheme will be called scheme I, and is not a part of the present invention.
Similarly, the flow scheme wherein "peeling" of a single pure component from a multicomponent mixture will be called scheme II. Scheme II is also not a part of the present invention.
The present invention will be called scheme III. It is an improvement over both of the prior art schemes in that it permits substantial savings in utility costs when compared to either scheme I or II.